Network Working Group J. Salowey
Internet-Draft H. Zhou
Expires: July 29, 2006 Cisco Systems
P. Eronen
Nokia
H. Tschofenig
Siemens
January 25, 2006
Transport Layer Security Session Resumption without Server-Side Statedraft-salowey-tls-ticket-07.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a mechanism which enables the Transport Layer
Security (TLS) server to resume sessions and avoid keeping per-client
session state. The TLS server encapsulates the session state into a
ticket and forwards it to the client. The client can subsequently
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Internet-Draft Stateless TLS Session Resumption January 20061. Introduction
This document defines a way to resume a Transport Layer Security
(TLS) session without requiring session-specific state at the TLS
server. This mechanism may be used with any TLS ciphersuite. This
document applies to both TLS 1.0 defined in [RFC2246] and TLS 1.1
defined in [I-D.ietf-tls-rfc2246-bis]. The mechanism makes use of
TLS extensions defined in [I-D.ietf-tls-rfc3546bis] and defines a new
TLS message type.
This mechanism is useful in the following types of situations:
1. servers that handle a large number of transactions from
different users
2. servers that desire to cache sessions for a long time
3. ability to load balance requests across servers
4. embedded servers with little memory
2. Terminology
Within this document the term 'ticket' refers to a cryptographically
protected data structure which is created by the server and consumed
by the server to rebuild session specific state.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Protocol
This specification describes a mechanism to distribute encrypted
session state information in the form of a ticket. The ticket is
created by a TLS server and sent to a TLS client. The TLS client
presents the ticket to the TLS server to resume a session.
Implementations of this specification are expected to support both
mechanisms. Other specifications can take advantage of the session
tickets, perhaps specifying alternative means for distribution or
selection. For example a separate specification may describe an
alternate way to distribute a ticket and use the TLS extension in
this document to resume the session. This behavior is beyond the
scope of the document and would need to be described in a separate
specification.
3.1 Overview
The client indicates that it supports this mechanism by including a
SessionTicket TLS extension in the ClientHello message. The
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extension will be empty if the client does not already possess a
ticket for the server. The extension is described in Section 3.2
If the server wants to use this mechanism, it stores its session
state (such as ciphersuite and master secret) to a ticket that is
encrypted and integrity-protected by a key known only to the server.
The ticket is distributed to the client using the NewSessionTicket
TLS handshake message described in Section 3.3. This message is sent
during the TLS handshake before the ChangeCipherSpec message after
the server has successfully verified the client's Finished message.
Client Server
ClientHello -------->
(empty SessionTicket extension)
ServerHello
(empty SessionTicket extension)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
NewSessionTicket
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
The client caches this ticket along with the master secret and other
parameters associated with the current session. When the client
wishes to resume the session, it includes the ticket in the
SessionTicket extension within ClientHello message. The server then
decrypts the received ticket, verifies that the ticket validity,
retrieves the session state from the contents of the ticket and uses
this state to resume the session. The interaction with the TLS
Session ID is described in Section 3.4. If the server successfully
verifies the client's ticket then it may renew the ticket by
including a NewSessionTicket handshake message after the ServerHello.
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ClientHello
(SessionTicket extension) -------->
ServerHello
(empty SessionTicket extension)
NewSessionTicket
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
A recommended ticket format is given in Section 4.
If the server cannot or does not want to honor the ticket then it can
initiate a full handshake with the client.
3.2 SessionTicket TLS extension
The SessionTicket TLS extension is based on [I-D.ietf-tls-
rfc3546bis]. The format of the ticket is an opaque structure used to
carry session specific state information. This extension may be sent
in the ClientHello and ServerHello.
If the client possesses a ticket that it wants to use to resume a
session then it includes the ticket in the SessionTicket extension in
the ClientHello. If the client does not have a ticket and it is
prepared to receive one in the NewSessionTicket handshake message
then it MUST include a zero length ticket in the SessionTicket
extension. If the client is not prepared to receive a ticket in the
NewSessionTicket handshake message then it MUST NOT include a
SessionTicket extension unless it is sending a non-empty ticket it
received through some other means from the server.
The server uses an zero length SessionTicket extension to indicate to
the client that it will send a new session ticket using the
NewSessionTicket handshake message described in Section 3.3. The
server MUST send this extension in the ServerHello if it wishes to
issue a new ticket to the client using the NewSessionTicket handshake
message. The server MUST NOT send this extension if it does not
receive on in the ClientHello.
If the server fails to verify the ticket then it falls back to
performing a full handshake. If the ticket is accepted by the server
but the handshake fails the client SHOULD delete the ticket.
The SessionTicket extension has been assigned the number TBD1. The
format of the SessionTicket extension is given at the end of this
section.
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struct {
opaque ticket<0..2^16-1>;
} SessionTicket;
3.3 NewSessionTicket handshake message
This message is sent by the server during the TLS handshake before
the ChangeCipherSpec message. This message MUST be sent if the
server included a SessionTicket extension in the ServerHello. This
message MUST NOT be sent if the server did not include a
SessionTicket extension in the ServerHello. In the case of a full
handshake, the server MUST verify the client's Finished message
before sending the ticket. The client MUST NOT treat the ticket as
valid until it has verified the server's Finished message. If the
server determines that it does not want to include a ticket after it
has included the SessionTicket extension in the ServerHello then it
sends a zero length ticket in the NewSessionTicket handshake message.
If the server successfully verifies the client's ticket then it MAY
renew the ticket by including a NewSessionTicket handshake message
after the ServerHello in the abbreviated handshake. The client
should start using the new ticket as soon as possible after it
verifies the Server's finished message for new connections. Note
that since the updated ticket is issued before the handshake
completes it is possible that the client may not put the new ticket
into use before it initiates new connections. The server MUST NOT
assume the client actually received the updated ticket until it
successfully verifies the client's Finished message.
The NewSessionTicket handshake message has been assigned the number
TBD2 and its definition is given at the end of this section. The
ticket_lifetime_hint field contains a hint from the server about how
long the ticket should be stored. The value indicates the lifetime
in seconds as a 32 bit unsigned integer in network byte order. A
value of zero is reserved to indicate that the lifetime of the ticket
is unspecified. A client SHOULD delete the ticket and associated
state when the time expires. It MAY delete the ticket earlier based
on local policy. A server MAY treat a ticket as valid for a shorter
or longer period of time than what is stated in the
ticket_lifetime_hint.
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struct {
HandshakeType msg_type;
uint24 length;
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
case session_ticket: NewSessionTicket; /* NEW */
} body;
} Handshake;
struct {
uint32 ticket_lifetime_hint;
opaque ticket<0..2^16-1>;
} NewSessionTicket;
3.4 Interaction with TLS session ID
If a server is planning on issuing a SessionTicket to a client that
does not present one it SHOULD include an empty Session ID in the
ServerHello. If the server includes a non-empty session ID then it
is indicating intent to use stateful session resume. If the client
receives a SessionTicket from the server then it discards any Session
ID that was sent in the ServerHello.
When presenting a ticket the client MAY generate and include a
Session ID in the TLS ClientHello. If the server accepts the ticket
and the Session ID is not empty then it MUST respond with the same
Session ID present in the ClientHello. This allows the client to
easily differentiate when the server is resuming a session or falling
back to a full handshake. Since the client generates a Session ID
the server MUST NOT rely upon the Session ID having a particular
value when validating the ticket. If a ticket is presented by the
client the server MUST NOT attempt to use the Session ID in the
ClientHello for stateful session resume. Alternatively, the client
MAY include an empty Session ID in the ClientHello. In this case the
client ignores the Session ID sent in the ServerHello and determines
if the server is resuming a session by the subsequent handshake
messages.
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Internet-Draft Stateless TLS Session Resumption January 20064. Recommended Ticket Construction
This section describes a recommended format and protection for the
ticket. Note that the ticket is opaque to the client so the
structure is not subject to interoperability concerns, so
implementations may diverge from this format. If implementations do
diverge from this format they must take security concerns seriously.
Clients MUST NOT examine the ticket under the assumption that it
complies with this document.
The server uses two different keys, one 128-bit key for AES [AES] in
CBC mode [CBC] encryption and one 128-bit key for HMAC-SHA1 [RFC2104]
[SHA1].
The ticket is structured as follows:
struct {
opaque key_name[16];
opaque iv[16];
opaque encrypted_state<0..2^16-1>;
opaque mac[20];
} ticket;
Here key_name serves to identify a particular set of keys used to
protect the ticket. It enables the server to easily recognize
tickets it has issued. The key_name should be randomly generated to
avoid collisions between servers. One possibility is to generate new
random keys and key_name every time the server is started.
The actual state information in encrypted_state is encrypted using
128-bit AES in CBC mode with the given IV. The MAC is calculated
using HMAC-SHA1 over key_name (16 octets)and IV (16 octets), followed
by the length of the encrypted_state field (2 octets) and its
contents (variable length).
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struct {
ProtocolVersion protocol_version;
CipherSuite cipher_suite;
CompressionMethod compression_method;
opaque master_secret[48];
ClientIdentity client_identity;
uint32 timestamp;
} StatePlaintext;
enum {
anonymous(0),
certificate_based(1),
psk(2)
} ClientAuthenticationType;
struct {
ClientAuthenticationType client_authentication_type;
select (ClientAuthenticationType) {
case anonymous: struct {};
case certificate_based:
ASN.1Cert certificate_list<0..2^24-1>;
case psk:
opaque psk_identity<0..2^16-1>;
}
} ClientIdentity;
The structure StatePlaintext stores the TLS session state including
the master_secret. The timestamp within this structure allows the
TLS server to expire tickets. To cover the authentication and key
exchange protocols provided by TLS the ClientIdentity structure
contains the authentication type of the client used in the initial
exchange (see ClientAuthenticationType). To offer the TLS server
with the same capabilities for authentication and authorization a
certificate list is included in case of public key based
authentication. The TLS server is therefore able to inspect a number
of different attributes within these certificates. A specific
implementation might choose to store a subset of this information or
additional information. Other authentication mechanisms, such as
Kerberos [RFC2712], would require different client identity data.
5. Security Considerations
This section addresses security issues related to the usage of a
ticket. Tickets must be sufficiently authenticated and encrypted to
prevent modification or eavesdropping by an attacker. Several
attacks described below will be possible if this is not carefully
done.
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Implementations should take care to ensure that the processing of
tickets does not increase the chance of denial of serve as described
below.
5.1 Invalidating Sessions
The TLS specification requires that TLS sessions be invalidated when
errors occur. [CSSC] discusses the security implications of this in
detail. In the analysis in this paper, failure to invalidate
sessions does not pose a security risk. This is because the TLS
handshake uses a non-reversible function to derive keys for a session
so information about one session does not provide an advantage to
attack the master secret or a different session. If a session
invalidation scheme is used the implementation should verify the
integrity of the ticket before using the contents to invalidate a
session to ensure an attacker cannot invalidate a chosen session.
5.2 Stolen Tickets
An eavesdropper or man-in-the-middle may obtain the ticket and
attempt to use the ticket to establish a session with the server,
however since the ticket is encrypted and the attacker does not know
the secret key, a stolen ticket does not help an attacker resume a
session. A TLS server MUST use strong encryption and integrity
protection for the ticket to prevent an attacker from using a brute
force mechanism to obtain the tickets contents.
5.3 Forged Tickets
A malicious user could forge or alter a ticket in order to resume a
session, to extend its lifetime, to impersonate as another user or
gain additional privileges. This attack is not possible if the
ticket is protected using a strong integrity protection algorithm
such as a keyed HMAC-SHA1.
5.4 Denial of Service Attacks
The key_name field defined in the recommended ticket format helps the
server efficiently reject tickets that it did not issue. However, an
adversary could store or generate a large number of tickets to send
to the TLS server for verification. To minimize the possibility of a
denial of service, the verification of the ticket should be
lightweight (e.g., using efficient symmetric key cryptographic
algorithms).
5.5 Ticket Protection Key Management
A full description of the management of the keys used to protect the
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ticket is beyond the scope of this document. A list of RECOMMENDED
practices is given below.
o The key should be generated securely following the randomness
recommendations in [RFC4086]
o The key and cryptographic protection algorithms should be at least
128 bits in strength
o The key should not be used for any other purpose than generating
and verifying tickets
o The key should be changed regularly
o The key should be changed if the ticket format or cryptographic
protection algorithms change
5.6 Ticket Lifetime
The TLS server controls the lifetime of the ticket. Servers
determine the acceptable lifetime based on the operational and
security requirements of the environments in which they are deployed.
The ticket lifetime may be longer than the 24 hour lifetime
recommended in [RFC2246]. TLS clients may be given a hint of the
lifetime of the ticket. Since the lifetime of a ticket may be
unspecified a client has its own local policy which determines when
it discards tickets.
5.7 Alternate Ticket Formats and Distribution Schemes
If the ticket format or distribution scheme defined in this document
is not used then great care must be taken in analyzing the security
of the solution. In particular if a confidential information, such
as a secret key, is transferred to the client it MUST be done using
secure communication so as to prevent attackers from obtaining or
modifying the key. Also the ticket MUST have its integrity and
privacy protected with strong cryptographic techniques to prevent a
breach in the security of the system.
5.8 Identity Privacy, Anonymity and Unlinkability
This document mandates that the content of the ticket is
confidentiality protected in order to avoid leakage of its content,
such as user relevant information. As such, it prevents disclosure
of potentially sensitive information carried within the ticket.
The initial handshake exchange, which was used to obtain the ticket,
might not provide identity confidentiality of the client based on the
properties of TLS. Another relevant security threat is the ability
for an on-path adversary to observe multiple TLS handshakes where the
same ticket is used and to therefore conclude that they belong to the
same communication endpoints. Application designers that use the
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ticket mechanism described in this document should consider that
unlinkability [ANON] is not necessarily provided.
While a full discussion of these topics is beyond the scope of this
document, it should be noted that it is possible to issue a ticket
using a TLS renegotiation handshake that occurs after a secure tunnel
has been established by a previous handshake. This may help address
some privacy and unlinkability issues in some environments.
6. Acknowledgments
The authors would like to thank the following people for their help
with preparing and reviewing this document: Eric Rescorla, Mohamad
Badra, Tim Dierks, Nelson Bolyard, Nancy Cam-Winget, David McGrew,
Rob Dugal, Russ Housley, Amir Herzberg, Bernard Aboba and members of
the TLS working group.
[CSSC] describes a solution that is very similar to the one described
in this document and gives a detailed analysis of the security
considerations involved. [RFC2712] describes a mechanism for using
Kerberos [RFC4120] in TLS ciphersuites, which helped inspire the use
of tickets to avoid server state. [I-D.cam-winget-eap-fast] makes
use of a similar mechanism to avoid maintaining server state for the
cryptographic tunnel. [SC97] also investigates the concept of
stateless sessions.
7. IANA considerations
IANA has assigned a TLS extension number of TBD1 (the value 35 is
suggested) to the SessionTicket TLS extension from the TLS registry
of ExtensionType values defined in [I-D.ietf-tls-rfc3546bis].
IANA has assigned a TLS HandshakeType number TBD2 to the
NewSessionTicket handshake type from the TLS registry of
HandshakeType values defined in [I-D.ietf-tls-rfc2246-bis].
8. References8.1 Normative References
[I-D.ietf-tls-rfc2246-bis]
Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.1", draft-ietf-tls-rfc2246-bis-13 (work in progress),
June 2005.
[I-D.ietf-tls-rfc3546bis]
Blake-Wilson, S., "Transport Layer Security (TLS)
Extensions", draft-ietf-tls-rfc3546bis-02 (work in
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